In an effort to protect vehicle operators and occupants, vehicle manufacturers and their suppliers are increasingly designing and manufacturing vehicles with additional and improved safety features. One such safety feature that has been incorporated into a number of vehicle types is anti-lock braking, which can take a variety of forms.
Vehicle anti-lock brake systems are designed to maximize the ability of a vehicle operator to bring a vehicle to a controlled stop on any type of road surface. An anti-lock brake system accomplishes this goal by preventing the vehicle brakes from prematurely halting vehicle wheel rotation, or "locking" the vehicle wheels, regardless of the road surface and the pressure applied to the brake pedal by the vehicle operator.
Typical vehicle anti-lock brake systems include vehicle wheel speed sensors for providing inputs to an anti-lock brake system control unit. The control unit controls anti-lock brake system control valves interposed between the brake master cylinder and the individual wheel brakes of an hydraulic brake circuit. Such control valves include isolation valves and reduction valves. The control valves, in turn, regulate hydraulic brake fluid pressure in the individual wheel brakes to implement anti-lock braking.
One known method utilized in identifying an anti-lock braking event includes comparing the velocity of one or more of the vehicle wheels to a reference vehicle velocity. The reference vehicle velocity is an estimate of the true vehicle velocity based on current and previous values of the individual wheel speeds. If the velocity of a wheel is significantly less than the vehicle reference velocity, then the wheel is deemed by the anti-lock brake system to be experiencing a "pre-lockup" condition. The anti-lock brake system then reduces the pressure activating the brake associated with that wheel in order to reduce brake torque. The reduction of brake torque allows the friction force at the surface to accelerate the wheel, thereby causing a reduction of a slip in the wheel.
A second known method of detecting an antilock braking event includes comparing the deceleration of each wheel to a deceleration threshold. The wheel deceleration is determined from the difference between the filtered wheel speed and raw wheel speed. By utilizing wheel deceleration in identifying an anti-lock braking event, a departure of wheel velocity from vehicle velocity will be identified sooner so as to limit the departure.
One problem associated with anti-lock brake systems as described above is their tendency toward false, or premature, activation. Premature activation can occur in a number of circumstances, such as where a road surface is partially ice-covered or bumpy. Such anti-lock brake systems activate when the departure depth of any one of the vehicle wheels exceeds the threshold, despite the fact that the wheel would not have experienced excessive slip.
Another problem associated with anti-lock brake systems as described above is their tendency to initiate drive train excitation. Drive train excitation can occur where a departure is deep and continues for a long period of time. Thus, the continuation of dumping and reapplication of brake pressure causes the engine to oscillate.